Such pipelines are used, for example, in a secondary metallurgical treatment of metal melts to introduce alloying and addition materials into a treatment vessel. Difficulties arise, in particular, when the treatment vessel is closed by a removable cover and/or has to be moved vertically. Both of these cases not only require a corresponding free space above the vessel but also prevent addition devices such as storage containers, locks for the introduction of solids under reduced pressure and metering devices from being located on the vessel or on the cover. This would in fact lead not only to considerably more space being needed, but especially to a quite considerable increase in weight, which would require correspondingly designed lifting or transport devices.
These problems are encountered, in particular, in the vacuum treatment of metal melts; thus for example in the RH process a vacuum vessel fitted with two suction pipes has to be raised before the treatment and then lowered over a ladle containing the melt to be treated until its two suction pipes dip into the melt in the ladle. The vacuum-lift or DH process requires a similar positioning of the vacuum vessel provided with a dipping pipe, which in addition is constantly raised and lowered during the vacuum treatment. Finally, in stand degassing a ladle containing the melt is introduced from a above into a vacuum vessel provided with a removable cover.
In all these cases it is necessary to keep the volume and the weight of the parts of the plant which need to be moved as small as possible, the more so as, with advances in secondary metallurgy and the increasing volume of the melts to be treated, the amounts of material to be added become greater and greater. This has led to the addition devices such as bunkers, locks and metering devices being fixed in place independently of the treatment vessel concerned usually at a higher level-and connected to the treatment vessel by way of pipelines.
In order to follow the movements of the treatment vessel and/or of a cover or to enable the space above the treatment vessel to be cleared it is known to use pipelines made up of lengths of pipe connected together telescopically. Such pipelines are able to take up the necessary movements of the vessel and/or of the cover, but they suffer from the disadvantage that as solids are introduced during the vacuum treatment air penetrates from the surroundings into the telescopic pipeline and consequently into the vacuum vessel, since it is impracticable to fit the individual lengths of pipe with seals which are a match for the reduced pressures of less than one mbar which are nowadays usual in intensive decarburisation. The reason for this is that the operating conditions in the secondary metallurgical treatment of metal melts are extraordinarily severe and the seals are not only exposed in part to considerable temperature stress but are also subjected to the abrasive wear caused by the more or less hard addition materials such as ferromanganese, ferrosilicon, ferrochromium, carbon, aluminum and limestone with a particle size of up to about 50 mm. To this is added the risk of formation of deposits of solids in the region where the walls of the lengths of pipe slide on one another.
The risk of deposits being formed also exists in the case of pipelines made in other ways, for example with elbows, particularly when sections of the pipeline are only slightly inclined to the horizontal and/or when the speed of transport of the solids is small.